In recent years, there have been many attempts to tackle environmental problems such as global warming, and there has also been demand for engine oils (lubricating oil compositions) to show a fuel economy effect. For instance, low viscosity lubricating oil compositions have been discovered in which the friction coefficient in the boundary lubrication domain is reduced by blending in organic molybdenum compounds (for example, see Japanese Laid-open Patent 2002-371292). Also, low viscosity lubrication oil compositions have been discovered in which the friction coefficient in the boundary lubrication domain is reduced by blending in organic molybdenum compounds and further in which a fuel economy effect is exhibited even in the hydrodynamic lubrication domain by blending in a lubricating oil base oil based on specific esters (for example, see Japanese Laid-open Patent 2005-041998).
Furthermore, low viscosity lubrication oils have been discovered in which it has been possible to exhibit a superior fuel economy effect even without blending in organic molybdenum compounds, by blending in combinations of specific anti-oxidants (for example, see Japanese Laid-open Patent 2005-146010). Also, as examples of commercial engine oils sold as fuel economising oils, mention may be made of low viscosity oils such as SAE viscosity grades 5W-30, 5W-20 and 0W-20 and low viscosity oils blended with organic molybdenum compounds.
Apart from these, lubricating oil compositions have been disclosed which, as well as containing organic molybdenum compounds, are blended so that a sulphur component is offered (for example, see Japanese Laid-open Patent H08-253785 (1996), Japanese Laid-open Patent 2004-149762 and Japanese Laid-open Patent H09-104888 (1997)). Lubricating oil compositions have also been disclosed which contain molybdenum compounds and dithiocarbamates (thiocarbamoyl compounds) (for example, see Japanese Laid-open Patent H10-121079 (1998) and Japanese Laid-open Patent H10-130680 (1998)).
Organic molybdenum compounds are chiefly categorised as three types: the so-called molybdenum dialkyldithiophosphates (which may be referred to below as MoDTPs), the so-called molybdenum dialkyldithiocarbamates (which may be referred to below as MoDTPs), and those which are compounds in which the molybdenum becomes an amine complex. Recently, MoDTPs, because they contain the element phosphorus, have hardly been used in lubricating oils for internal combustion engines. This is because, if engine oils with MoDTPs added are used in actual engine oils, whenever trace amounts of engine oil enter the combustion chamber via compression rings or via engine valves and are burnt together with the fuel, exhaust gases containing elemental phosphorus derived from the MoDTPs are emitted, and there is concern that they will have a detrimental effect on the apparatus for treating the emissions and on the life of the catalyst.
On the other hand, MoDTCs do not contain phosphorus, and so are used as friction modifiers in lubricating oils for internal combustion engines. It is known that MoDTCs form a film on the sliding friction surfaces inside the engine and said film contains “molybdenum disulphide” compounds in which the elemental composition ratio is close to molybdenum disulphide. MoDTCs contain sulphur and molybdenum in their molecules and so they break down on a sliding friction surface and a film containing molybdenum disulphide compounds is formed. These molybdenum disulphide compounds are believed to reduce friction.
MoDTCs contain sulphur and molybdenum in their molecules, but the amount of sulphur relative to the amount of molybdenum is comparatively small, and so it has not been easy to form the molybdenum disulphide compounds satisfactorily with the MoDTCs alone. In order to enhance the activity of film formation (formation of the molybdenum disulphide compounds), it has been necessary to increase the sulphur component. For this reason, a sulphur component has been supplied from outside (for example, see Japanese Laid-open Patent H08-253785 (1996), Japanese Laid-open Patent 2004-149762 and Japanese Laid-open Patent H09-104888 (1997)). However, increasing the sulphur component is not desirable, because it accelerates clogging of the emissions treatment apparatus and poisoning of the catalyst. As far as practicable, it is necessary to obtain a bigger friction reducing effect by reducing the supply of a sulphur component from outside.
Likewise, compounds in which molybdenum has formed an amine complex do not have sulphur in their molecules, and so find it difficult to create molybdenum disulphide compounds beyond the MoDTCs. For this reason, the friction reducing effect becomes extremely small. In the case of amine complexes, therefore, it has been absolutely necessary to supply a sulphur component from outside.
This invention has been made in consideration of the aforesaid problems. The aim has been to minimise the effects on emission treatment apparatus and catalysts by reducing as far as practicable the amounts of MoDTCs and sulphur compounds contained, and so to offer, even where these may still be present in trace amounts, a lubricating oil composition which has a superior friction reducing effect over a long period and which has a high degree of fuel economy.